A traveling wave tube (TWT) is an amplifier that increases the gain, power or some other characteristic of a microwave or radio frequency (RF) signal, that is, electromagnetic waves typically within a range of around 0.3 GHz to above 300 GHZ. An RF signal to be amplified is passed through the device, where it interacts with and is amplified by an electron beam. The TWT is a vacuum device through which the electron beam travels, typically focused by a magnetic containment field to prevent the electron beam from directly touching the structure of the TWT.
The electron beam may be generated at the cathode of an electron gun, which is heated to typically about 1000 degrees Celsius. Electrons are emitted from the heated cathode by thermionic emission and are drawn through the TWT to a collector by a high voltage bias, focused by the magnetic field.
The TWT also contains a slow wave structure (SWS) such as a wire helix through which the RF signal passes. For example, in the case of the wire helix TWT, the electron beam passes through the central axis of the helix without significantly contacting or touching the inner walls of the helix. The slow wave structure is designed so that the RF signal travels the length of the TWT at about the same speed as the electron beam. As the RF signal passes through the slow wave structure, it creates an electromagnetic field that interacts with the electron beam, bunching or velocity-modulating the electrons in the beam. The velocity-modulated electron beam creates an electromagnetic field that transfers energy from the beam to the RF signal in the slow wave structure, inducing more current in the slow wave structure. The RF signal may be coupled to the slow wave structure and the amplified RF signal may be decoupled from the slow wave structure in a variety of ways, such as with directional waveguides that do not physically connect to the slow wave structure.
A number of different slow wave structures are known for use in traveling wave tubes, such as the wire helix TWT mentioned above, with corresponding advantages and disadvantages. For example, a wire helix TWT has a wide bandwidth, meaning that the RF signals that can be amplified in the wire helix TWT are less bandwidth-limited and may have a wider range of frequencies than in some other TWT designs. However, a wire helix TWT has some limitations when compared with other TWT designs. Another type of TWT is a coupled cavity TWT, in which the slow wave structure has a series of cavities coupled together. As the RF signal passes through the resonant cavities, inducing RF voltages in each cavity. When the velocity modulation of the electron beam passing adjacent the cavities is in phase, the RF voltages in each subsequent cavity increase in an additive fashion, amplifying the RF signal as it passes through the coupled cavity TWT. However, coupled cavity TWTs are often difficult to manufacture and assemble, including a large number of tiny components that must be precisely aligned and spaced. Although coupled cavity TWTs have relatively high gain, they also generally have narrower bandwidths than some other designs such as a wire helix TWT, leaving room for improvement in areas such as bandwidth and ease of construction.